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GENTHOD, Switzerland - Iran and six world powers put nuclear talks back on track Thursday at a landmark session that included the highest-level bilateral contact with the U.S. in years and a pledge to meet again this month. President Barack Obama then challenged Tehran to make good on its promises quickly.

Iran also agreed to allow U.N. inspectors into its covertly built enrichment plant during the talks, held at a villa outside Geneva. The discussions appeared to defuse tensions that had been building for weeks.

Speaking in Washington, Obama called the talks "a constructive beginning" and said Iran must match its words with action.

Tehran "must grant unfettered access" to international inspectors within two weeks, he said, warning that if Iran fails to follow through, "then the United States will not continue to negotiate indefinitely and we are prepared to move towards increased pressure."

"Our patience is not unlimited," Obama said. "Going forward, we expect to see swift action."

The tone of Thursday's meeting was considerably more positive than just a week ago, when the U.S. and its allies were threatening Iran with tough new sanctions if it refused to freeze its nuclear activities, which they suspect are aimed at creating an atomic weapon.

Perhaps the most significant development of the day was a 45-minute one-on-one meeting between U.S. Under Secretary of State William Burns and Iran's senior nuclear negotiator, Saaed Jalili. It was the first direct U.S. negotiations with Iran since Washington severed relations in 1980.

The encounter appeared to add to the positive atmosphere that led to agreement by all the parties -- Iran, the U.S., Britain, France, Russia, China and Germany -- for a follow-up meeting this month.

It also appeared to be concrete proof of Obama's commitment to engage Iran directly on nuclear and other issues -- a sharp break from policy under the Bush administration.

However, statements made by the two sides reflected the continuing divide between them.

U.S. Deputy State Department spokesman Robert Wood said Burns used the meeting with Jalili "to reiterate the international community's concerns about Iran's nuclear program."

"He addressed the need for Iran to take concrete and practical steps that ... will build international confidence in the exclusively peaceful nature of its program," he said.

Wood said both sides also "had a frank exchange on other issues, including human rights." Officials in Washington said Burns urged Tehran to resolve the cases of three Americans detained in Iran since July.

Javier Solana, the European Union's foreign policy chief, told reporters Iran agreed to "co-operate fully" with the International Atomic Energy Agency and to open its newly disclosed nuclear facility to inspectors, probably within "a couple of weeks."

In a statement, the IAEA said agency chief Mohamed ElBaradei had been invited to Iran to discuss nuclear issues. A senior U.S. official said elBaradei would travel to Tehran this weekend. He spoke on condition of anonymity because his information was confidential.

ElBaradei recently said Tehran was "on the wrong side of the law" over its new enrichment plant near the Shiite holy city of Qom. He said Iran should have revealed its plans as soon as it decided to build the facility.

Western officials said Iran also agreed to send some of its enriched uranium to Russia to further process the material for use in a research reactor in Tehran. It was a long-sought compromise because Iran has repeatedly refused to involve an outside country, insisting it has the right to a full domestic enrichment program.

Obama said such a step would help build international confidence.

"We support Iran's right to peaceful nuclear power," Obama said. "Taking the step of transferring its low-enriched uranium to a third country would be a step toward building confidence that Iran's program is in fact peaceful."

Still, questions remained over the way forward.

The six powers remain committed to seeking a full freeze of Tehran's nuclear activities, but Iran could argue that the agreement amounted to an acknowledgment of its right to enrich uranium.

Curbing Iran's ability to enrich uranium is a key international goal, because the process can produce both fuel for nuclear reactors and weapons-grade uranium for warheads.

The differences reflected the likelihood of huge bumps ahead in any future talks.

Secretary of State Hillary Rodham Clinton, speaking in Washington, sounded a pragmatic note. "Today's meeting opened the door, but let's see what happens," she said.

Iran's refusal to freeze its enrichment activities has already prompted three sets of U.N. Security Council sanctions.

Iran came to the talks with a proposal that ignored the key demand that it freeze enrichment. Instead it offered to hold "comprehensive, all-encompassing and constructive" discussions on a range of security issues, including global nuclear disarmament.

Reiterating calls by President Mahmoud Ahmadinejad, the Iranian offer linked any talks with a discussion of Middle East tensions "to help the people of Palestine achieve all-embracing peace."

It called for "reform" of the U.N. Security Council -- shorthand for curbing the authority of the U.S. and the four other permanent council members. The only link to the arms issue was a call for discussion of disarmament by the world's nuclear powers.

Jalili told reporters that while those issues were key, Thursday's discussions were "good talks" compared to the last seven-nation meeting 15 months ago that broke up in failure.

At the United Nations, Iranian Foreign Minister Manouchehr Mottaki suggested the talks could be expanded to the "summit" level. He said Iran was willing to discuss a variety of security, economic and political issues, although he did not specifically refer to nuclear issues.